Low-frequency Sound Absorption Based On Helical Artificial Structures

From the superposed sound pressure in the standing wave tube,we can extract the total sound pressure and particle velocity fields at any position in the tube,as well as the ratio of the maximum and minimum sound pressures,namely,the standing wave ratio.In experiments,the acoustic absorption coefficient of the load in the tube can be calculated from the standing wave ratio.The acoustic absorption coefficient of the load can be expressed in terms of the surface impedance.When the surface impedance matches the air impedance,complete sound absorption is realized.Due to the viscosity of air in the tube,there exists a velocity gradient near the tube wall.Hence,the particle velocity at the tube wall is set to be 0 m/s,which gradually increases towards the center axis.There also exist viscous damping losses in the boundary layer with a particle velocity gradient.When the structure operates at the local resonance,the acoustic energy is mainly dissipated through viscous losses and thermal losses.The sound absorbing structures in this thesis are based on local resonances.The resonance principle of the Helmholtz resonators is analyzed.The expression of acoustic impedance is obtained by analyzing the motion of air column in the tube resonator.When the surface impedance is matched with the air impedance,we can obtain the resonance frequency from the zero surface reactance.The maximum absorption coefficient can be obtained by setting the surface resistance match with air.In practice,the designed Helmholtz resonator achieves perfect sound absorption at 78 Hz.Here we propose a three-dimensional helical structured absorber with the impedance characteristic similar to the straight rectangular tub.The three-dimensional helical structured absorber has a high space utilization and provides a new approach for low-frequency ultrathin sound absorber.In the design,the thickness of the helical structured absorber is 52 mm and the frequency at perfect sound absorption is 207 Hz.Based on the resonance absorption in acoustic metasurface,we proposed a judicious design of tunable and multiple-band perfect absorbers with nesting helical tracks and deepsubwavelength thicknesses(<?/30 and ? is the operation wavelength).By rotating the cover cap with an open aperture on the nesting helical tracks,we can tailor the effective lengths of resonant tubular cavities in the absorber flexibly,with the absorption peak frequency shifted in a wide spectrum and the acoustic impedance all along matched with air.The simulated particle velocity fields at different configurations reveal that the sound absorption mainly occurs at the open aperture.Our experiment measurements agree well with the theoretical analysis and simulation,demonstrating the wide-spectrum and tunable absorption performance of the designed flat acoustic device.